The present invention relates to a method for producing a one-piece woven (OPW) air bag having single-layer and two-layer portions, an upper and lower outer surface and at least one coating applied portionally.
One-piece woven (OPW) air bags are known, comprising one or more portions whose outer upper and lower surface is coated, for example with a film. The film serves the purpose of preventing escape of a fluid or gas from the chamber formed in the multi-layer portion between the layers. Although the applied coatings are practically completely impermeable, the air bags as described above are not sufficiently fluid-tight. Because of this, the useful life of air bags, particularly of side and head air bags as demanded by industry, is difficult to attain or only by highly expensive generators which recharge the escaped gas in several stages or compensated by an added amount of propellant to equalize the gas loss. Apart from this the air bags as just described have the drawback that gas leakages can gain access to the interior of the vehicle making use of such an air bag possibly a health problem, due to the gas as such or its high temperature.
The invention is based on the object of proposing a method for producing an air bag which avoids or at least greatly diminishes the drawbacks of prior art. It has been surprisingly discovered that leakage of air bags is caused by a gas permeability of the single-layer fabric edging the air bag in a seam-like connection. What happens here is that the gas escapes from the chambers of the air bag through the single-layer, i.e. in the plane of the fabric located between the coatings of the upper and lower outer surface. The problem forming the basis of the invention is optimally solved by sealing the single-layer portion so that it is impermeable to fluids.
In one advantageous aspect of the invention the single-layer portions are provided at least in part with a fabric structure which is wovingly less dense than the remaining fabric. This now makes it possible for an applied coating, for example a film or laminate to penetrate deeper into the less densely woven fabric of the single-layer portion, and possible so deeply that the coatings come into contact in the interior of the single-layer portion in thus forming a “horizontal” closure which seals the single-layer portion of the fabric plane. This now makes it practically impossible for inflation gas to escape from the chambers formed by the two-layer portion.
A similar advantage is featured by another aspect of the invention wherein a low-viscosity coating is applied. This, similar to the method as just described, permits very deep penetration of the coating into the fabric in the single-layer portion in thus achieving complete sealing with the advantages as just described above. A likewise advantageous aspect of the invention materializes from a method in which the single-layer portion is thermally sealed. Thus, in this case by heating the edging, fusion of the upper and lower coating is achieved at the outermost edging of an air bag, again resulting in a substantially complete closure or seal of the single-layer portion.